Quantum Well Infrared Photodetector Focal Plane Arrays (QWIP FPA""s) are conventionally used for infrared detection and imaging. Typical applications of QWIP FPA""s include fiber optics communications systems, temperature sensing, night vision, eye-safe range finding, and process control. As is known in the art, QWIP FPA""s are composed of arrays of detector structures, wherein each detector structure produces a signal that is transmitted through a conductor bump to an external Read Out Integrated Circuit (ROIC) unit cell. The outputs of the plurality of ROIC unit cells associated with each detector in the array produce an integrated representation of the signal from the detector. To produce this output signal, a fixed bias is applied to the detector and the detector photocurrent resulting from the bias and the incident radiation is integrated. This integration function is performed by an integration charge well (integration capacitor) that is disposed within each individual ROIC unit cell. The combined integrated outputs of the plurality of ROIC unit cells in the array produce an image corresponding to the received infrared radiation.
As shown in FIG. 1, a conventional photodetector architecture consists of the detector structure 1 physically separated from the ROIC unit cell structure 2 and electrically connected through a conductive bump 3. In this prior art photodetector architecture the integrating capacitor 4 (unit cell charge well) is physically disposed within the ROIC unit cell itself. The usable area of the ROIC unit cell is constrained by the pitch of the overlying detector in the array. This constraint on the usable area further limits the size of the charge well which is the largest component in the unit cell. As the pitch of each detector in the array is reduced to create greater detector density, the usable area of the associated unit cell must also be reduced. This reduction limits the size of the charge well and, ultimately, places limits on the density of the detector array.
To produce very high density (for example 856xc3x97480 or 1024xc3x97576) QWIP Focal Plane Arrays the detector pitch may need to be reduced to less than 18 xcexcm. As already noted, this small pitch will limit the usable area for the unit cell charge well and also any additional unit cell storage well capacitors. Even with the use of 0.5 xcexcm or 0.35 xcexcm technology the available area for these capacitors in the unit cell will be very small. Additionally, there is a need to place smart focal plane array functions into the ROIC but in conventional designs, the small pitch limits the space that is available to provide these functions. Thus, the conventional photodetector architecture, in which the unit cell contains all of the components except the detector, imposes a limitation on the size and functionality of the unit cell charge well and the density of the FPA.
An additional drawback of the prior art concerns the inherent variation in the conductivities of each of the detectors in a focal plane array. These variations in conductivity result in detector elements that have different responsivities to incident radiation (e.g., high responsivity/xe2x80x9chotxe2x80x9d or low responsivity/xe2x80x9ccoldxe2x80x9d detector pixels). Variation in responsivity among the detectors across the array disadvantageously leads to nonuniform array imagery. However, the ROIC circuitry of the prior art fails to provide any compensation for this variation in responsivity. The conventional photodetector of FIG. 1 includes a ROIC injection transistor 26 that is used to bias the detector element 1. This transistor functions to provide a constant bias voltage that produces a linear response from the detector element 1. Since the detector""s responsivity is also a function of the applied bias voltage, the fixed bias provided by the prior art injection transistor 26 does not compensate for the variation in responsivity due to inherent variations in detector conductivity.
Another drawback of the prior art concerns the use of QWIP FPA""s as infrared target searching and tracking sensors. Such sensors are often required to possess low Noise Equivalence Irradiance (NEI) to distinguish faint targets near the background irradiance levels and high instantaneous dynamic range to prevent saturation while tracking intense targets. Conventional approaches to this problem include using automatic gain control (AGC) or increasing the charge well size and the detector""s A/D resolution. AGC solutions are appropriate when merely considering a single infrared target. Using AGC, the sensor initially can be operated in a high gain mode to acquire the target. As the infrared target grows in intensity, the sensor gain is reduced to prevent saturation. Use of AGC to change system gain, however, has the consequence that, if the system gain is adjusted to accomodate a high intensity IR target, the system sensitivity will be decreased such that other low intensity targets may not be acquired. In a specific application such as missile warning, this inability to acquire multiple targets is unacceptable.
Other conventional approaches to this problem include the use of larger charge wells and adding bits to the sensor A/D converter. Future sensors will, however, increasingly require multi-color, high resolution detectors. Such sensors will thus require small unit cell sizes and more complex ROICs. Charge well size will therefore eventually be constrained by space limitations. Additionally, the resolution of the sensor""s receiving electronics will likely be limited to 14 bits based on a number of factors including required data rates, the desire for a minimum number of outputs per channel, and component availability.
It would thus be desirable to provide an improved quantum well photodetector that provides solutions to the above identified problems, including an improved dynamic range for detecting multiple IR targets over a wide range of target irradiance. An additional aspect of the invention also provides for correction in the responsivity of the detector due to inherent variations in detector conductivity. This is accomplished by providing dynamic detector biasing. One way of doing this is by coupling the charge well to the detector. Another aspect of the invention consists of an improved detector structure where the charge well is fabricated on the detector itself. Fabricating the charge well on the detector structure may be accomplished by, for example, adding an extra contact layer and a dielectric layer to a standard quantum well such as a multiple quantum well. In one embodiment the added contact layer is composed of doped GaAs and the dielectric layer is composed of undoped GaAs. The dielectric layer can alternatively be constructed of other materials such as SiO, SiO2, ZnS, or MgF2. Fabricating the charge well on the detector structure permits a larger charge well as compared to the prior art and further permits configuration of the detector in a very high density focal plane array.